With current advancements in communication technologies, video sharing and streaming are increasingly becoming popular among Internet users in applications like social-aware P2P video transmission. Peer-to-Peer (P2P) paradigm was proven to be the promising approach to efficiently deliver streaming services over the Internet. Several P2Pbased streaming systems were successfully deployed. In such systems, the basic streaming quality parameters that affect the user Quality of Experience (QoE) are Playback Continuity and Startup Delay. Several research conducted have revealed that buffer filling strategies used in these systems play a key role on the streaming quality. The popularly used strategies are the Rarest First (RF) and Greedy (Gr) strategies, however, studies have shown that RF strategy outperforms Gr strategy in terms of playback continuity and Gr strategy outperforms RF strategy in terms of startup delay. In this paper, a problem of finding an optimal buffer filling strategy in form of mixed strategy was considered. The idea of the mixed strategy is to divide the buffer into two parts via demarcation point and apply one strategy to fill one part of the buffer and then the other strategy is applied to fill the other part of the buffer. Among the derived mixed strategies, an optimal was discovered which provides high probability of playback continuity with low startup delay. © Springer Nature Switzerland AG 2020.